1. Field of the Invention
This invention relates to methods for treating landfills for recovering methane therefrom.
2. Prior Art
Anaerobic digestion of waste materials in a landfill produces a mixture of gases comprised primarily of carbon dioxide and methane. It is desirable to recover the methane from the landfill for use as a fuel source. Also, it is desirable to remove the methane from the landfill for the reason that it represents a serious safety hazard in the event that it seeps out of the landfill into a closed structure such as a building.
It is known to withdraw the gas mixture from the landfill and bring it into contact with one side of a membrane more permeable to carbon dioxide than to methane under a pressure sufficient to cause the carbon dioxide to permeate the membrane to the other side thereof and thereby become separated from the methane. There are also other methods of removing the carbon dioxide to thus upgrade the fuel value of the landfill-generated gas. These methods, such as absorption of the carbon dioxide in a solvent (amine scrubbing) or adsorption of the carbon dioxide on a solid material (e.g. molecular sieves or activated carbon) also produce high purity methane for use as a fuel. This upgrades the methane to a quality fuel, which can be substituted for or added to normal pipeline natural gas. Also, this substantially eliminates the safety hazard which can be encountered when the gas mixture leaks out of the landfill.
All of the normally practical techniques, i.e., membranes, absorption or adsorption, all suffer from the same limitation. That is, if in withdrawing the gas from the landfill, air is allowed to enter the landfill and contaminate the normal carbon dioxide/methane mixture, the nitrogen cannot effectively be removed and will become a contaminant in the product such that the product cannot be used as a fuel. Also, the oxygen in the air will retard or even stop the anaerobic digestion of organic waste materials in the landfill.
In order to prevent air form entering the landfill around the periphery of the earth cover over the landfill, it is known to install spaced wells around the periphery of the landfill and then, using a reduced pressure, withdraw gas from these wells and vent it to atmosphere. This will withdraw any air which has seeped into the periphery of the landfill but is wasteful in that the gas streams removed from the peripheral wells will contain a significant quantity of methane which is lost.
In enhanced oil recovery carbon dioxide is forced into a well under very high pressures, these pressures usually being sufficiently high that the carbon dioxide would normally be in a liquid state, to cause the carbon dioxide to travel through oil bearing formations to dissolve and carry to another well the oil being recovered.